The present invention concerns a galvanic isolation device.
There are many requirements for galvanic isolation. They are dictated by various considerations:
functional considerations such as the need to carry out measurements at potentials different than that of the processing electronics within a complex system,
safety of persons: floating neutral,
electrical supply availability: impedance grounded neutral,
safety of persons and equipment: high common mode rejection due to atmospherically generated overvoltages or to work on distribution lines,
safe operation: rejection of common mode voltages and currents in the high throughput transmission of binary data.
These requirements are usually well met by current solutions essentially based on the magnetic circuit of a voltage or current transformer or on optical fibre technology.
The transformer is a known and simple means of obtaining galvanic isolation of an alternating current signal referred to a potential which can have any value but which is usually ground potential, a frame ground potential or a fixed potential.
The alternating current signal to be isolated may convey electrical energy from a generator to a receiver or data or energy and data simultaneously.
The transformer is rated for a given frequency or for a frequency band but has the disadvantage that it is not of itself able to transmit a direct current signal and the quality of its operation at low frequencies is generally limited by its dimensions.
The transformer has the advantage of being reversible, however, and of enabling electrical energy to flow in both directions, in other words from either winding to the other in the case of a transformer with two windings.
Also, it accommodates any type of receiver: inductive, capacitive or resistive, which means that it caters for any type of current/voltage phase difference and, for each polarity of an alternating current voltage applied to it, an instantaneous current in any direction and depending in practice only on the load.
Finally, provided that it is adequately rated, it preserves the incident electrical waveform. Thus an alternating current voltage amplitude modulated by a signal conveying data can be transmitted without significant distortion from one winding to the other using the magnetic coupling between the two windings.
Another known way to isolate two electrical circuits is to convert the electrical signal to an electromagnetic signal and then to carry out the reverse conversion.
The intermediate data medium is an electromagnetic wave of higher frequency than the incident electrical signal and which can be in the radio frequency domain or in the domain of visible or invisible light. The electromagnetic wave may be free or guided (by an optical fibre, for example).
The drawback of this method is that it provides no simple and economical way of transporting significant energy to power (for example) a complete electronic system for converting the electromagnetic signal into an electrical signal and then amplifying the resulting electrical signal. Also, a direct current signal can only be transmitted with some uncertainty as to its level, given temperature and time drift in the electrical, electromagnetic and electromagnetic/electrical conversion yield and variation in losses in the transmission medium. To alleviate this drawback a direct current signal must undergo frequency conversion.
As shown in FIG. 1, it is also possible to associate with a transformer 1 an electronic switching circuit ("chopper") 2 and rectifier and filter circuits 3 enabling a direct current signal conveying energy to be transferred from the primary of the transformer to the secondary.
This amounts to frequency conversion in the energy domain.
Any data that may be superimposed on the direct current signal is then simultaneously sampled--in the digital signal processing sense--by the chopper and reconstituted by the rectifier/filter provided that the switching/sampling frequency is at least twice the highest frequency to be transmitted in the frequency spectrum of the signal conveying the data (SHANNON's theorem).
The chopper which chops the direct current signal and samples any superimposed data signal can use components of any technology: GTO thyristors, MOS or bipolar transistors, IGBT, vibrators, etc.
The combination of the chopper and the transformer can also use any available technique: forward, push-pull, flyback, resonant or non-resonant.
Depending on which technique is used, the transformer may or may not have an additional auxiliary demagnetisation winding.
This principle can achieve electrical isolation of a signal:
carrying energy or not, for example to supply power to downstream circuits,
carrying data or not,
having a transmit frequency spectrum extending from zero frequency (direct current) to half the switching/sampling frequency.
Note, however, that the electronic switches used in the chopper are not reversible, that is to say that they cannot chop a current flowing in the opposite direction. If the component enables the reverse flow of current (asymmetric thyristors, MOS transistors, etc) they cannot block it (reverse-bias diodes). Where these components allow simultaneously reverse current flow and blocking of the current, performance is usually very much reduced, as in the case of the bipolar transistor. What is more, the rectifier diode(s) connected to the transformer secondary winding determine a particular direction for the secondary current. These two considerations imply a given direction of the voltage and the current at the primary and at the secondary, ruling out the use of this principle in respect of a voltage whose polarity changes with time; this is a first limitation.
A second limitation results from the fact that the chopper is on one side of the transformer and the rectifier-filter on the other side. This arrangement determines a direction of energy flow as it requires control of the magnetizing current of the transformer on the same side as the chopper, which is associated with the supply.
The use of this principle therefore introduces a two-fold lack of symmetry, in respect of:
the polarity of the voltage and the current,
the direction of energy flow.
There are therefore situations in which no acceptable solution has been found to the problem of galvanic isolation, for the theoretical or technological reasons explained above; this applies when the galvanic isolation must cater for direct current electrical signals or electrical signals likely to include a direct current component and must further be symmetrical with regard to the direction of energy flow, in other words reversible; this is a fortiori the case where the galvanic isolation must also be symmetrical with regard to the polarity of the applied signal.
The techniques outlined above provide no economical means of achieving, for example, electrical isolation of analog or digital telecommunication terminals receiving a remote power feed, and in particular rule out simultaneous implementation of the following functions:
transmission of energy by direct current or at 50 Hz (or 60 Hz) or at 50 Hz (or 60 Hz) superimposed on a direct current,
two-way transmission of data in a wide frequency band: from direct current to frequencies of several hundred kilohertz, for digital transmission at 144 kbauds or from direct current up to 12 kHz, 36 kHz or even 60 kHz for analog transmission,
polarity changes at the isolation input transferrable to the output in the case of a 50 Hz or 60 Hz ringing current, alone or superimposed on a direct current signal whose amplitude is below the peak voltage of the 50 or 60 Hz signal in the case of an analog link; this change of polarity may be the simple interchanging of the two non-dedicated and non-polarized DC power wires in the case of an analog or digital link,
reversibility of the instantaneous direction of power flow at 50 or 60 Hz in an analog terminal calling phase to enable operation on any type of load, especially a capacitive load, and thus to authorize the exchange of reactive energy between the load and the supply,
remote power feed to the electrical circuits of the isolating system itself; for this the efficiency of the isolation must be excellent, especially on standby.